Tuesday, April 13, 2010

Estimating Turf Water Use, Part 1

By Michael D. Vogt, CGCS, CGIA

To understand how much water a turf plant requires is often a matter of intuition on the role of the golf course superintendent. Often that last inspection of the day is the deciding factor on how much water to apply to the turf that evening. With the advent of computer operated irrigation systems a percent increase here and decrease there is all that’s needed. Once the system is “dialed in” that’s the basic truth to the majority of irrigation scheduling. That’s one aspect of the art of greenkeeping, right?

Water can be applied with a degree of science as well as experience and the following is the step by step way to increase your technical knowledge on the science of water application on turfgrass. Whether you use these techniques or not it is critical to know for the simple reason that someday someone’s going to ask, “How did you arrive at the amount of water needed to keep the turf healthy with over watering?”

Evapotranspiration

The amount of water used by a section of turf on a golf course over any given period of time depends on local weather conditions, soil moisture availability and the characteristics of the turf species. Turf water use is also affected by the hydrogeological characteristics of the site and the infiltration rates of the soil. Soil infiltration rates can be measured with single or double ring infiltrometer.

One way to quantify the water needs of a particular type of turf is to identify its Plant Water Requirements (PWR). The PWR is the amount of water needed by the turf for growth, including the water lost through evapotranspiration (ET). ET is the amount of water transferred to the atmosphere by evaporation from soil and plant surfaces, plus the amount of water vapor released through the plant stomata via transpiration. For most golf course turf surfaces, transpiration is much greater than evaporation and therefore makes up the vast majority of ET.

Note: Confusion in the use of the term “ET” often exists. There are two industry accepted definitions of ET- the potential and the actual ET. Potential ET (sometimes given as “PET”) is defined as the ET rate that will occur for a given weather condition for “well watered grass”. Actual ET is equal to potential ET except where soil moisture is limiting, in which case actual ET is less than potential ET. Since the potential ET is of principal interest in determining turf water needs for irrigation, and the term “ET” is used in the industry to refer to potential ET, the term “ET” as used in this writing will mean potential ET unless otherwise specified.

Estimating Evapotranspiration (ET)

The most important factors contributing to ET rates are solar radiation, air temperature, wind speed and atmospheric moisture. Both local meteorology and soil characteristics can vary tremendously within an area the size of a typical golf course. The south side of an elevated area, with greater exposure to wind and radiation will have a greater potential ET rate than a slope with a northern exposure. Consistently, shaded areas will have lower ET rates than areas in full sun. These fine-scale variations in the physical environment are referred to as “microclimate.” ET rates calculated using regional weather data may provide a general indication of potential water use, but they should be adjusted up or down depending on the microclimates present in an individual golf course.

Double Ring Infiltrometer

Soil moisture availability is greatly influenced by soil type and texture. Sandy soils have high porosities but drain readily and do not have high available water holding capacities. Loam soils have the highest water holding capacities, whereas clay soils, although relative high in water contents, hold water so tightly that plants cannot remove the water for transpiration at lower water contents. A low area lying closer to the water table will require less irrigation than an area higher in the landscape because of upward flow of water (capillary rise) into the root zone from the water table, especially for the sandy loam soils.

There are many methods of estimating ET. Some of the more common approaches include obtaining data from outside sources, measuring ET, and calculating ET.

1. Outside sources of ET data. ET estimates can be obtained from commercial weather monitoring and forecasting operations. There are also publicly available weather data sets that often include estimates of ET. Values are usually given as a daily rate in mm per day or inches per day and are based on either evaporation pan data or an equation that estimates ET. This data is usually intended to describe conditions at a regional scale, and may over-or underestimate local conditions.

2. Measuring ET On Site. An alternative to using outside, regional ET estimates is the installation of one or more weather stations to measure on site ET. This alternative would be indicated, for example, when regional weather stations have been shown to consistently misrepresent local conditions. Some devices include:

a. On site weather stations (mostly directly connected into the irrigation system central computer).

b. Class A Evaporation pans. A U.S. Weather Service Class A evaporation pan is 122 centimeters in diameter and 25 centimeters deep and is supported 15 centimeters above the ground. The pans are filled with water and the amount of water that evaporates from the pan roughly correlates to the amount of water lost from turf due to evapotranspiration. The amount is not exactly the same; more water usually evaporates from the pan than is lost from the turf. A crop coefficient for evaporation pan data (Kc) is applied to the evaporation pan measurements to arrive at ET rates.

c. ET gages or Atmometers. These devices have a water reservoir connected by a wicking device to a surface such as a porous plate that mimics a leaf surface. The amount of water lost from the reservoir represents the ET for the given weather conditions. Rates will be less than from an evaporation pan since there is some resistance to flow through the wicking material. These are relatively inexpensive and should be located in the various microclimates found on the course.

3. Calculating ET. Regional weather operations and some measurement devices estimate ET using theoretical physical equations. These equations use available weather measurements, and normally make some assumptions with respect to local soil conditions and the nature of the plant canopy. It may be possible to obtain more accurate ET estimates by using local weather data, then adjusting the parameters of the ET equation to reflect the characteristics of the specific soil and vegetation present on the golf course.

1. Penman equation. This equation, often referred to as the Modified Penman equation, provides an estimate of evaporation from a free water surface. Four weather variables are required for this equation, solar radiation, wind, temperature and humidity. It is often used in place of pan evaporation. Since Penman and others have found that the equation also predicts well the ET from a 3-6" tall cool season grass that completely covers the ground, and is supplied with adequate water, it is sometimes referred to as a reference ET (ETo). A crop coefficient (Kc) for whatever species of grass is being irrigated is applied to the equation to get an estimate of the potential rate of ET for that crop.

2. Penman-Monteith equation. This equation predicts the ET from a crop directly.
The same four weather variables are required as the Penman equation plus a canopy conductance term that accounts for resistance to water movement within the reference plant. The specific canopy conductance values for individual crops are not commonly available; therefore, the Penman-Monteith equation is not used in practice as frequently as the modified Penman equation.

4. Blaney-Criddle. This equation was originated for use in the Western United States.
It uses temperature and day-length as the major independent variables for estimating ET. There are crop coefficients specific to the Blaney-Criddle equation available in the U.S. Soil Conservation Service (1970) handbook. It is recommended that the Blaney-Criddle equation be used for monthly ET estimation. This equation is simple but provides only a rough estimate. It may produce large errors under extreme weather conditions, especially outside of the Western United States where it was developed (Dunne and Leopold, 1978).

Crop Coefficients and Species Specific Water Use Rates

In addition to the physical environmental factors discussed above, the amount of water used by a turf canopy will also depend on the nature of the canopy itself. Within a species, water use needs vary diurnally and seasonally, and depend on the stage of development of the grass. “Crop coefficients” are a useful way of expressing relative water use efficiency numerically.

1. Species and Cultivar variations. Water use needs also vary among species, and cultivars of particular grass species can also vary in their water use rates. Warm season grasses tend to have lower water use rates while cool season grasses have higher rates. This is partly because cool season grasses use ET as a cooling mechanism.

Some turf species can have a lower comparative PWR and still require more water to maintain an acceptable quality than a species with a higher PWR. This is because some species have greater drought tolerance than others. The goal is to use grass species or cultivars that have a lower PWR and a high drought tolerance. A study by Aronson et al.

2. Crop Coefficients. Crop coefficients, as mentioned in the explanations above, are ratios of the potential ET of a particular crop, species or cultivar to a reference ET or evaporation rate. These coefficients are determined experimentally, often using weighing lysimeters under “unlimited soil water” conditions. Care must be used in the use of crop coefficients as the term is used for various references, Blaney-Criddle, Penman-Monteith, pan evaporation, and Penman evaporation equation. Crop coefficients will vary with the species of grass in question, the growth stage of the plants, the climate, the season, cutting height, and soil moisture stress, arriving at a single number to use as a crop coefficient can be problematic. Most golf course superintendents use crop coefficients in conjunction with experience to arrive at a consistent coefficient for their turf location and varieties.

A study of crop coefficients in the Northeast is the study by Aronson et al. This study compared measured ET rates for several species or cultivars with both pan evaporation and values predicted by the modified Penman equation. As studied the Penman equation and pan evaporation, rates varied both seasonally and from year to year. The authors concluded that using an averaged Kc value of 1.0 for the cool season turfgrass species studied would be adequate for irrigation scheduling. These values are higher than the typical values for turf of approximately 0.7 to 0.8.

Brown et al. (2001) in a study in Arizona found Kc values ranging from about 0.75 to 0.85 for Bermuda grass. These Kc values are for the Penman-Monteith equation for potential ET, not evaporation, and therefore would be expected to higher rather than lower than values based on the modified Penman equation or pan evaporation. A study by Carrow (1995) found that an average coefficient for tall fescue in the southeast for summer would be (0.79-0.82). This study also found the coefficients for turfgrass differed over the growing season.

Plant Available Water

Plant Available Water (PAW) represents the quantity of water stored within the root zone between the conditions of field capacity and the permanent wilting point. To solve for PAW as simple equation is this:

PAW = AW x RZ

PAW = Plant Available Water (Inches)

AW = Available Water (Inches of Water / Inches of Soil

RZ = Root Zone Depth

Management Allowed Depletion

Management allowed depletion (MAD) is the maximum amount of plant available water (PAW) expressed as a percent that the superintendent allows to be removed from the soil before irrigation occurs.

A value of 50% MAD is a reasonable overall value before irrigation will occur. Although, shallow root zones and sensitive plants such as Poa annua will require irrigation before the 50% threshold is reached.

Allowable Depletion

Now that we know the PAW and MAD we can apply an equation to arrive at the allowable depletion (AD):

AD = PAW x MAD

For example:

If the MAD is 50% (0.50) and the PAW is 2.2 inches, then the approximately 1.10 inches of water could be used by the turf before the next irrigation (2.2 x 0.50 = 1.10 inches).

The next installment will discuss run time multipliers and how to figure run times based on these physical statistics.